Exposure apparatuses for semiconductor processing are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes an illumination source, a mask stage assembly that positions a reticle, an optical assembly, a wafer stage assembly that positions a semiconductor wafer, a measurement system, and a control system. Typically, the wafer is divided into a plurality of rectangular shaped integrated circuits.
There are two kinds of exposure apparatuses that are generally known and currently used. The first kind is commonly referred to as a Stepper lithography system. In a Stepper lithography system, the reticle is fixed (except for slight corrections in position) and the wafer stage assembly moves the wafer to fixed chip sites where the illumination source directs an illumination beam at an entire reticle pattern on the reticle. After the exposure, the wafer is moved (“stepped”) to the next site for subsequent exposure. In this type of system, the throughput of the apparatus is largely governed by how quickly the wafer stage assembly accelerates and decelerates the wafer between exposures during movement between sites.
The second kind of system is commonly referred to as a Scanner lithography system. In a Scanner lithography system, the mask stage assembly moves the reticle concurrently with the wafer stage assembly moving the wafer during the exposure process. With this system, the illumination beam is slit shaped and illuminates only a portion of the reticle pattern on the reticle. With this design, only a portion of the reticle pattern is exposed and transferred to the site on the wafer at a given moment, and the entire reticle is exposed and transferred to the site on the wafer over time as the reticle pattern is moved through the exposure slit. After the entire site is scanned, (i) the wafer stage assembly decelerates the wafer, steps the wafer, and subsequently accelerates the wafer in the opposite direction during movement of the wafer to the next site, and (ii) the mask stage assembly decelerates the reticle and subsequently accelerates the reticle in the opposite direction so that the reticle is moving in the opposite direction during the exposure of the next site. In this type of system, there is one stepping motion for every scanning motion. Thus, throughput of the apparatus is largely governed by how quickly the wafer stage assembly accelerates and decelerates the wafer, and how quickly the mask stage assembly accelerates and decelerates the reticle.
There is a never ending search to increase the throughput in terms of exposures per hour for the exposure apparatuses. With the current exposure apparatuses, assuming that there is sufficient light to adequately expose the wafer, in order to gain higher throughput, it is necessary to move the wafer and/or reticle at higher speeds, and accelerations. Unfortunately, it is not always easy to merely increase the velocities and accelerations of the wafer and the reticle.